PPP, DNS: introduce ppp_set_usepeerdns macro

PPP use peer DNS setting is currently hardcoded to true if PPP is
acting as a client and set to false if PPP is actinf as a server.

This is probably the most wanted behavior, but since we now have the
ability to change that at runtime, allow users to do it.

We don't have a way to have a different default configuration if the
PPP PCB is going to be used as a client or as a server, therefore the
default configuration should be fine for both of them. Since enabling
peer DNS by default is dangerous for server mode, the default is now
not to ask for DNS servers and it should now be explicitely enabled
if needed, update the documentation accordingly.

src/include/netif/ppp/ppp.h

@@ -466,6 +466,12 @@ void ppp_set_auth(ppp_pcb *pcb, u8_t authtype, const char *user, const char *pas
  * for PPP server support.
  */
 #define ppp_set_ipcp_dnsaddr(ppp, index, addr) (ppp->ipcp_allowoptions.dnsaddr[index] = ip4_addr_get_u32(addr))
+
+/*
+ * Whether we ask the peer for up to 2 DNS server addresses. Received DNS server addresses are
+ * registered using the dns_setserver() function. Default is false.
+ */
+#define ppp_set_usepeerdns(ppp, boolval) (ppp->settings.usepeerdns = boolval)
 #endif /* LWIP_DNS */
 #endif /* PPP_IPV4_SUPPORT */